This invention relates generally to circuit breakers and more particularly to a movable contact arm assembly for a circuit breaker with a blow open contact arrangement.
Current limiting circuit breakers are advantageous to use because they can increase the interrupting current rating of a circuit breaker without appreciably increasing the size of the circuit breaker. Circuit breakers are constantly being improved to increase interrupting current rating, reduce size or to reduce the manufacturing cost. The user of a circuit breaker is genuinely concerned about the physical size of the circuit breaker, the electrical and thermal ratings of the circuit breaker and, of course, the cost of the circuit breaker. Accordingly, it can be appreciated that it would be highly desirable to provide a circuit breaker which makes efficient use of space, has an increased current carrying capability and is relatively easy to manufacture.
The blow open circuit breaker increases the interrupting current rating of the circuit breaker without appreciably increasing the overall size of the unit when compared to the size that a traditional unit would have for the same current interrupting capability. Although current limiting circuit breakers are well established in the work place, there is a never ending need for increased current interrupting capacity in the same physical space. As wonderful as the current limiting circuit breaker with blow open contacts may be, there are problems. One problem is that to interrupt higher currents in the confined space, the speed of the contact opening action must be sufficiently fast that an arc is properly drawn and the contacts do not weld shut. Also, at higher current ratings, greater contact pressure when the contacts are closed is needed to minimize the contact resistance and the heat generated thereby.
To insure adequate contact closing pressure, springs have been used to hold the contacts closed. Upon the occurrence of a fault or over current condition, the contacts open against the force of the spring. Quite understadably, having to work against the force of the spring slows the opening action of the circuit breaker which limits its current interrupting capability. Accordingly, it can be appreciated that it would be highly desirable to provide a circuit breaker which provides adequate spring closing force yet can react quickly to open when a fault occurs.
Past attempts to overcome the problem of balancing the spring closing force with the fast opening of the contacts have employed the use of various springs and cams and roller arrangements which mechanically reduce the spring closing force as the contacts begin to open. The spring force acting to keep the contacts closed is reduced from a maximum value when the contacts are closed to some lesser value as the contacts are opened. There is always a closing force acting on the contacts urging them toward the closed position. Accordingly, it will be appreciated that it would be highly desirable to provide a spring force to maintain the contacts in a closed position which would not hinder the opening of the contacts.
Accordingly, it is an object of the present invention to provide a movable contact arm assembly for a circuit breaker.
Another object of the present invention is to provide a movable contact arm assembly which has a positive closing force acting thereon.
Yet another object of the present invention is to provide a movable contact assembly which, after overcoming initial spring closing force, opens freely.